Ok Time for me to chime in also

A 40LA on an LT40 will be just about right although a 46AX is better on that plane. A 70FS on the 4*60 is gonna be marginal at best. Plan on a long takeoff run. A 70FS on a 4*40 is WAY over powered and I doubt the airframe can take it for any length of time. Yes the 4* series are strong but over power them and you will see issues related to the firewall and front former where the wing dowels go into. The very two places that you do not want anything bad to happen

Now, if you are still having problems with orientation I HIGHLY recommend you go back on the trainer. I cannot stress this enough. Everyone has orientation problems at times but let me ask you these questions. I do all of these with my students before I turn them loose.

1.) Can you tell what the airplane is doing at all times
2.) Can you tell where the airplane is at all times
3.) can you do mulitple loops/rolls/stall turns all without losing altitude on the loops and rolls and follow the exact same line coming down as you did going up in the stall turn
4.) Can you fly inverted for at least three full patterns around the field maintaining altitude and track.
5.) Can you keep your plane reasonably straight on takeoff
6.) Can you land the plane in reasonably the same spot whenever you want
7.) if the plane is flying directly at you and someone tells you to turn right, can you do it every time, mostly without thinking about it (reworded below)
8.) How often do you grease your landings

If you can do all of the above with very little thought them move up to the next plane. If not stay with the trainer. Most every trainer, especially with the LT-40 can do all of the above and more.

I know allot of the above sounds harse but knowing and being able to do everything I listed will help you with that Cub and will make the transition to the second plane easier.

BTW, This is going to be a kit, so if you go with what is listed on tower for a 65 -70 four stroke then build extremely light and strong